ISAAC/VLT observations of a lensed galaxy at z= 10.0*

We report the first likely spectroscopic confirmation of a $z\sim 10.0$galaxy from our ongoing search for distant galaxies with ISAAC/VLT. Galaxy candidates at $z \ga 7$are selected from ultra-deep JHKs images in the core of gravitational lensing clusters for which deep optical imaging is also available, including HST data. The object reported here, found behind Abell 1835, exhibits a faint emission line detected in the Jband, leading to $z=10.0$when identified as Lyα, in excellent agreement with the photometric redshift determination. Redshifts $z < 7$are very unlikely for various reasons we discuss. The object is located on the critical lines corresponding to $z=9$to 11. The magnification factor μranges from 25 to 100. For this object we estimate ${\it SFR}({\rm Ly}\alpha) \sim (0.8{-}2.2)~M _{\odot}$yr-1and ${\it SFR}({\rm UV}) \sim (47{-}75)~M _{\odot}$yr-1, both uncorrected for lensing. The steep UV slope
indicates a young object with negligible dust extinction. SED fits with young low-metallicity stellar population models yield (adopting $\mu=25$) a lensing corrected stellar mass of $M_\star \sim 8 \times 10^6~M _{\odot}$, and luminosities of $2 \times 10^{10}~L _{\odot}$, corresponding to a dark matter halo of a mass of typically $M_{\rm tot} \ga 5 \times 10^8~M _{\odot}$. In general our observations show that under excellent conditions and using strong gravitational lensing direct observations of galaxies close to the “dark ages” are feasible with ground-based 8-10 m class telescopes.